The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:                3GPP third generation partnership project        ABS almost blank subframes        ACK acknowledgement        CCE control channel element        CSG closed subscriber group        D2D device-to-device        DCI downlink control information        eICIC enhanced inter cell interference coordination        eNB evolved Node B        HARQ hybrid automatic repeat request        NACK negative acknowledgement        PDCCH physical downlink control channel        PDSCH physical downlink shared channel        PUCCH physical uplink control channel        RAR random access response        RNTI radio network temporary identifier        RRC radio resource control        TA timing advance        UE user equipment        
The radio environment has become more complex as different systems overlap and the need has arisen to coordinate among them for smart-phones and other types of user equipments which communicate on multiple systems at once. Traditional hierarchical cellular arrangements are generically termed a macro network or macro cell, and within or near that macro cell is one or more other radio sub-environments such as a femto cell (operating what is sometimes termed an underlay network) or device-to-device communications. Such overlapping networks are often referred to as heterogeneous networks. The available radio spectrum is most efficiently employed when there is some coordination among these different radio networks.
FIG. 1 is a schematic diagram of such a heterogeneous network. There is a UE 20 in communication with a macro eNB 22 in a conventional cellular arrangement. Nearby is a femto eNB 26 which is assumed to operate on at least some of the same frequency bands as the macro eNB 22, and/or is another UE#2 with which the first UE 20 is engaging in D2D communications directly. Currently it is assumed the best effective management of this radio environment centers on the macro eNB 22, for it can coordinate its transmissions so as not to interfere with the lower transmit power of the femto eNB 26 and can allocate specific radio resources for the UEs to use for their D2D communications. The end goal is to avoid interference as much as practical between the conventional cellular communications on link 21, communications with the femto eNB 26 on link 23, and the D2D communications on link 27.
LTE Release 10 (LTE-A) has introduced a mechanism to mitigate interference between links 21 and 23, termed enhanced inter-cell interference coordination eICIC. In this technique the macro eNB 22 and the femto eNB 26 coordinate to avoid interfering transmissions. Specifically, the macro eNB 22 will restrict itself in certain identified almost-blank subframes (ABS) to transmit nothing except the common reference signals used for measurements (and in some cases also essential control information like synchronization, paging, or system information) but never any unicast DL user data. During these ABSs transmissions by the femto eNB 26 are ‘protected’ in that transmissions from the macro eNB with its greater transmit power will not interfere with the lower power femto eNB transmissions. If UE 20 is not attached to the femto eNB 26 it can measure the common reference signal which the macro eNB 22 transmits in the ABS and report its radio link measurement to the macro eNB 22 for mobility purposes.
As to the D2D link 27, Qualcomm, Inc. has proposed evolving the LTE platform in order to intercept the demand of proximity-based applications by studying enhancements to the LTE radio layers that allow devices to discover each other directly over the air, and potentially communicate directly. See for example documents Tdoc-RP-110706 entitled ON THE NEED FOR A 3GPP STUDY ON LTE DEVICE-TO-DEVICE DISCOVERY AND COMMUNICATION; Tdoc-RP-110707 entitled STUDY ON LTE DEVICE TO DEVICE DISCOVERY AND COMMUNICATION—RADIO ASPECTS; and Tdoc-RP-110708 entitled STUDY ON LTE DEVICE TO DEVICE DISCOVERY AND COMMUNICATION—SERVICE AND SYSTEM ASPECTS (all from 3GPP TSG-RAN Meeting #52 plenary; Bratislava, Slovakia; 31 May-3 Jun. 2011) [1-3]. These proposals include a radio level discovery functionality, which needs also to be coupled with a system architecture and a security architecture that allow the 3GPP operators to retain control of the UE behavior (for example, to control who can emit discovery signals, when and where, what information these signals should carry, and what actions the corresponding UEs should take once they discover each other).
Basics of the eICIC concept are outlined above and these have also been discussed extensively for RRC-IDLE mode UEs in past 3GPP sessions, including where the idle mode UE 20 roams near to a femto cell 26 which is a closed subscriber group CSG cell for which this particular UE 20 does not have access and so is not allowed to initiate communication towards that CSG cell 26. FIG. 1 illustrates this with a one-way femto link 23; the UE 20 is forced to communicate with the overlaying macro cell 22, despite that it may experience severe interference from the CSG cell 26 or initiate the inter frequency measurements for cell re-selection purposes. It is not yet resolved how the UE 20 in RRC IDLE mode could inform the macro cell 22 about the situation of severe interference as early as possible when initiating communication towards that macro cell 22. Once the macro cell 22 was aware of the interference issue it could protect the UE 20 using the ABSs of the femto cell 26, but until then the UE 20 will suffer interference between links 21 and 23.